Currently, most heat-resistant alloys utilized in industry are either nickel-based alloys or steels with high nickel content (e.g., austenitic steels). These contain a delicate balance of various alloying elements, such as chromium, cobalt, niobium, tantalum and tungsten, to produce a combination of high temperature strength, ductility and resistance to attack in the environment of use. These alloying elements also affect the fabricability of components, and their thermal stability during use. Although such alloys have been used extensively in past, they do not meet the requirements for use in components such as those in advanced fossil energy conversion systems. The main disadvantages are the high material costs, their susceptibility to aging embrittlement, and their catastrophic hot corrosion in sulfur-containing environments.
In contrast, binary iron aluminide alloys near the Fe.sub.3 A1 composition have certain characteristics that are attractive for their use in such applications. This is because of their resistance to the formation of low melting eutectics and their ability to form a protective aluminum oxide film at very low oxygen partial pressures. This oxide coating will resist the attack by the sulfur-containing substances. However, the very low room temperature ductility (e.g., 1-2%) and poor strength above about 600 degrees C are detrimental for this application. The room temperature ductility can be increased by producing the iron aluminides via the hot extrusion of rapidly solidified powders; however, this method of fabrication is expensive and causes deterioration of other properties. The creep strength of the alloys is comparable to a 0.15% carbon steel at 550 degrees C; however, this would not be adequate for many industrial applications.
Considerable research has been conducted on the iron aluminides to study the effect of compositions to improve the properties thereof for a wider range of applications. Typical of this research is reported in U.S. Pat. No. 1,550,508 issued to H. S. Cooper on Aug. 18, 1925. Reported therein are iron aluminides wherein the aluminum is 10-16%, and the composition includes 10% manganese and 5-10% chromium. Other work is reported in U.S. Pat. No. 1,990,650 issued to H. Jaeger on Feb. 12, 1935, in which are reported iron aluminide alloys having 16-20% Al, 5-8.5% Cr, 0.4-1.5% Mn, up to 0.25% Si, 0.1-1.5% Mo and 0.1-0.5% Ti. Another patent in the field is U.S. Pat. No. 3,026,197 issued to J. H. Schramm on Mar. 20, 1962. This describes iron aluminide alloys having 6-18% Al, up to 5.86% Cr, 0.05-0.5% Zr and 0.01-0.1%B. (These two references do not specify wt% or at.%.) A Japanese Pat. (No. 53119721) in this field was issued on Oct. 19, 1978, to the Hitachi Metal Company. This describes iron aluminide alloys, for use in magnetic heads, in wt% of 1.5-17% Al, 0.2-15% Cr and 0.1-8% of "alloying" elements selected from Si, Mo, W, Ti, Ge, Cu, V, Mn, Nb, Ta, Ni, Co, Sn, Sb, Be, Hf, Zr, Pb, and rare earth metals.
Two typical articles in the technical literature regarding the iron aluminide research are "DO.sub.3 -Domain Structures in Fe.sub.3 Al-X Alloys" as reported by Mendiratta, et al., in High Temperature Ordered Alloys, Materials Research Society Symposia Proceedings, Volume 39 (1985), wherein various ternary alloy studies were reported involving the individual addition of Ti, Cr, Mn, Ni, Mo and Si to the Fe.sub.3 Al. The second, by the same researchers, is "Tensile Flow and Fracture Behavior of DO.sub.3 Fe-25 At.% Al and Fe-31 At.% Al Alloys", Metallurgical Transactions A, Volume 18A, Feb. 1987.
Although this research had demonstrated certain property improvements over the Fe.sub.3 Al base alloy, considerable further improvement appeared necessary to provide a suitable high temperature alloy for many applications. For example, no significant improvements in room temperature ductility or high temperature (above 500 degrees C) strength have been reported. These properties are especially important if the alloys are to be considered for engineering applications. It should also be noted that additives in the form of other elements may improve one property but be deleterious to another property. For example, an element which may improve the high temperature strength may decrease the alloy's susceptability to corrosive attack in sulfur-bearing environments.
Accordingly, it is an object of the present invention to provide an alloy having a composition near Fe.sub.3 Al that has improved room temperature ductility.
It is another object to provide such an alloy that has sufficient strength at high temperatures so as to be useful for structural components.
Another object is to provide such an alloy that is resistant to deleterious attack in environments containing sulfur compounds.
A further object is to provide such an alloy that is resistant to aging embrittlement.
These and other objects of the present invention will become more apparent upon a consideration of the full description of the invention as set forth hereinafter.